I think this comment, "Because in these types of situations, the temptation to do what comes naturally always looms." is more a reflection on the poster than the world in general. I disagree with Ricker's comment, but it isn't too much worse other than being explicit rather than discretely worded. One could also write a similar statement about women being seductresses etc. which would have the same set of problems.
What comes naturally to many people is to be friends, enjoy the person's company, have conversation, participate in platonic activities that both enjoy, etc. If the only thing that one thinks comes naturally between men and women is scrabble, then I think that's a sorry state of affairs.
From an etiquette point of view, if you can't talk to your bf or gf about your relationship, something is just wrong. I would say it's rude to accuse them of cheating or trying to cheat without clear evidence, but not wrong to say something makes you uncomfortable. For example, if you said, "I can't believe you want to go help her, I just know you'll jump right into bed with her" that seems rude and not productive. On the other hand, saying, "You going to spend the night makes me uncomfortable, can we talk about it" is perfectly fine.